U.S. patent number 5,077,975 [Application Number 07/519,191] was granted by the patent office on 1992-01-07 for control for a load-dependently operating variable displacement pump.
This patent grant is currently assigned to Mannesmann Rexroth GmbH. Invention is credited to Wolfgang Kauss.
United States Patent |
5,077,975 |
Kauss |
January 7, 1992 |
Control for a load-dependently operating variable displacement
pump
Abstract
Disclosed is a preferably electrohydraulic control for a
load-dependently operating variable displacement pump, which is
equipped with a differential pressure control device for ensuring a
predetermined pressure drop at the control edge of a directional
valve integrated into the consumer supply line. The differential
pressure control device has a servo valve, the valve spool of which
can be brought out of a first position, increasing the output
volume of the variable displacement pump, against the compressive
force of a spring and of a control pressure led off from the supply
pressure upstream of the directional valve, by the pump pressure
into a second position, reducing the output volume. In order to be
able to adapt the volume flow control to the different requirements
of various consumers, in particular of a mobile piece of equipment,
such as for example a farming tractor, and thereby improve the
quality of the control, the control pressure effective at the servo
valve is arbitrarily influenced by means of a separately
controllable actuating device, preferably designed as a servo
valve.
Inventors: |
Kauss; Wolfgang (Lohr-Wombach,
DE) |
Assignee: |
Mannesmann Rexroth GmbH
(Lohr/Main, DE)
|
Family
ID: |
6380193 |
Appl.
No.: |
07/519,191 |
Filed: |
May 4, 1990 |
Current U.S.
Class: |
60/452; 60/445;
60/446; 60/449 |
Current CPC
Class: |
C01B
3/508 (20130101); F04B 1/324 (20130101); F15B
11/055 (20130101); F04B 49/08 (20130101); F16H
61/433 (20130101); F15B 2211/20553 (20130101) |
Current International
Class: |
C01B
3/50 (20060101); C01B 3/00 (20060101); F04B
49/08 (20060101); F04B 1/12 (20060101); F04B
1/32 (20060101); F16D 031/02 () |
Field of
Search: |
;60/452,449,445,450,446,448 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0010860 |
|
May 1980 |
|
EP |
|
1801137 |
|
Apr 1970 |
|
DE |
|
3001167 |
|
Jul 1981 |
|
DE |
|
3044515 |
|
Jun 1982 |
|
DE |
|
3208250 |
|
Sep 1983 |
|
DE |
|
3410071 |
|
Oct 1985 |
|
DE |
|
3413913 |
|
Oct 1985 |
|
DE |
|
3805287 |
|
Aug 1989 |
|
DE |
|
Other References
Backe, W.: Neue Moglichkeiten der Verdrangerregelung, o+u
Olhydraulik und Pneumatik 32, 1988, Nr. 11, pp. 778-783..
|
Primary Examiner: Look; Edward K.
Assistant Examiner: Mattingly; Todd
Claims
What is claimed is:
1. Control for a load-dependently operating variable displacement
pump, having a differential pressure control device for ensuring a
predetermined pressure drop at at least one control edge of a
directional valve integrated into the consumer supply line, the
differential pressure control device having a servo valve, the
valve spool of which can be brought out of a first position
increasing the output volume of the variable displacement pump,
against the compressive force of a spring and of a control pressure
led off from the supply pressure upstream of the directional valve
into a second position, reducing the output volume, wherein the
control pressure effective at the servo valve can be influenced
arbitrarily by means of a separately controllable actuating device
and wherein the actuating device is formed by a valve means, the
valve means having an outlet connection to a line carrying the
control pressure and an inlet connection for a line carrying the
pump pressure as well as a tank connection.
2. Control according to claim 1, wherein the directional valve,
which in a neutral position closes all connections, and the control
pressure is led off via an orifice from the respectively highest
load pressure in the lines leading to a consumer downstream of the
proportional directional valve, the effective pressure between
orifice and servo valve being variable by means of the actuating
device.
3. Control according to claim 1 or 2 wherein the actuating device
is formed by a 3/3-way directional valve.
4. Control according to claim 3, wherein in a neutral position of
the directional valve positions, either a pressure relief or a
pressure pre-loading of the outlet connection takes place.
5. Control according to claim 4, wherein the directional valve is a
preferably electrically activated proportional directional
valve.
6. Control according to claim 5, wherein the control signal of the
proportional directional valve is dependent on the speed of the
variable displacement pump.
7. Control according to claim 5 wherein the spool valve of the
proportional directional valve can be centered in the neutral
position against the force of a spring.
8. Control according to claim 2 wherein the directional valve,
integrated into the consumer supply line, is electrically
activated.
9. Control according to claim 1 or 2 or 8 wherein the servo valve
of the differential pressure control device is a 3/2-way servo
valve which, in the first position indreases the output volume of
the variable displacement pump, relieves to the tank a line leading
to a working chamber of the variable displacement pump and, in the
second position connects this line to the pump line, while at the
same time blocking the tank connection.
10. Control according to claim 9, wherein the line leading to the
working chamber of the variable displacement pump is lead via a
pressure limiting valve, which admits pump pressure to this line
and blocks a connection to the 3/2-way servo valve if a set pump
maximum pressure is exceeded.
11. Control according to claim 10, wherein the pressure limiting
valve is formed by a 3/2-way directional valve, the valve spool of
which can be displaced out of a flow position, connecting the
connection to the working chamber of the variable displacement pump
to a connection leading to the 3/2-way servo valve, against the
force of a spring by the pump pressure into the other position in
which a pump pressure connection to the working chamber is switched
through.
12. Control according to claims 1 or 2 or 8 wherein the variable
displacement pump serves to supply consumers which are operated in
a closed control loop.
13. Control according to claims 1 or 2 or 8 wherein the influencing
of the control pressure effective at the servo valve preferably
takes place automatically, dependent on the respective operating
state of the consumer.
14. Control according to claims 1 or 2 or 8 wherein the control is
in mobile commercial vehicles, such as for example, farming
tractors or the like.
15. Control according to claim 6 wherein the spool valve of the
proportional directional valve can be centered in the neutral
position against the force of a spring.
Description
BACKGROUND OF THE INVENTION
The invention relates to a control for a load-dependently operating
variable displacement pump.
Such a control is used in the case of so-called load-sensing
hydraulic systems in order to control the volume flow delivered by
the variable displacement pump according to the actual requirements
of the consumers to be supplied. For this purpose, it is necessary
to keep the pressure drop at the control edge or the control edges
of the directional valve connected upstream of the at least one
consumer, which valve is frequently designed as a proportional
directional valve or servo valve, at a predetermined value, i.e. to
set a control pressure drop or a control pressure difference which
is effective at the consumer. Serving this purpose is the
differential pressure control device which, by constant or
continuous relieving and loading of an actuating element of the
variable displacement pump adapts the volume flow of the latter to
the actual consumer requirements.
The invention is based on such a load-sensing variable displacement
pump control loop, which is diagrammatically represented by the
prior art shown, for example, in U.S. Pat. No. 4,011,721, and in
FIG. 1.
In FIG. 1, the reference symbol 2 denotes a variable displacement
pump, which is driven for example by the engine of a mobile piece
of equipment, such as for example a farming tractor, and feeds a
consumer supply line or pump line 4, which is led to a consumer
(not shown in any more detail), such as for example a power lift, a
working piston of the steering, to the trailer braking system or to
a control unit of the suspension hydraulics. The consumer supply
line 4 is led via a directional valve 6, designed here as a 5/3-way
proportional directional valve, which has an inlet connection 8, a
tank connection 10, two working connections 12 and 14 as well as a
connection 16 for a load-indicating line 18. The load-indicating
line 18 is connected to a control surface 20 of a differential
pressure controller 30, which is designed as a 3/2-way servo valve.
The other control surface 22 is pressurized via a control line 24
by the pump pressure P in the consumer supply line 4 and can be
displaced out of the position A shown by the force of the pressure
acting on the control surface 22 against the force of a control
spring 26, which adds to the force of the pressure occurring via
the load-indicating line 18 at the control surface 20, into the
position B, in which a tank connection 28 is blocked and a control
pressure line 34 leading to a working chamber 32 of the variable
displacement pump 2 is connected to a further control line 36,
carrying the pump pressure P, in order to influence the variable
displacement pump 2 in the sense of a controlled cutting-back of
the output volume. 40 denotes a further servo valve, which can be
displaced out of a position C, switching through the control
pressure line 34, against the force of a restoring spring 42 by the
coming into effect of the pump pressure P acting in a control line
44, into the position D, in which a control pressure line branch
34A is blocked and, via a further control line 46, a pressurization
of the control pressure line 34 with the pump pressure P in the
consumer supply line 4 takes place.
The proportional directional valve 6 has three main switching
positions E, F and G, it being possible for the spool valve to be
centred in the neutral position F by the springs 48 and 50 and
displaced by suitable electric or hydraulic activation into the two
other switching positions E and G, in order to effect a reversal of
the admission of pressure to the working connections 12 and 14. In
the switching positions E and G, the connection 16 of the
load-indicating line 18 is in each case connected to the inlet
connection 8, so that the pressure effective at the consumer
prevails in the load-indicating line 18. In the neutral position F,
the connection 16 of the load-indicating line is connected to the
tank connection T, so that the load-indicating line 18 is relieved.
Upon displacement of the proportional directional valve 6 out of
the neutral position F, a control pressure drop occurs at the
control edge of this valve, the set value of which drop is
predetermined by the force of the control spring 26 of the
differential pressure controller 30. In the neutral position F, in
which the load-indicating line 18 is switched to the tank, the
variable displacement pump 22 goes into so-called standby
operation, the control spring 26 in this case determining the
standby pressure. The servo valve 40 acts in the sense of a
pressure limitation on the varying mechanism of the pump 2, i.e. on
the working chamber 32, if the pump pressure P exceeds a maximum
pressure. This limit pressure can be predetermined by the spring
42.
A pump controlled in this way is used in mobile pieces of
equipment, such as for example farming tractors, bulldozers or the
like, for supplying a wide variety of consumers, such as for
example the steering, the power lift, the trailer braking system,
the suspension hydraulics and other additional control units, which
however also invariably make different requirements on the volume
flow control. For instance, steering, power lift and trailer
braking system in each case operate in master closed control loops,
on which high requirements are made regarding speed of response and
stability. The various switching functions of the suspension
hydraulics additionally require the provision of relatively large
volume flows in a short time. Often the entire pump output volume
must be made available to the additional control units.
BRIEF DESCRIPTION OF THE INVENTION
To be taken into consideration as a further important boundary
condition in the case of such pump controls is that the speed of
response of the control must be in effect even at very low motor
speeds, in particular with a view to an adequate steering and
switching function of the suspension hydraulics. On the other hand,
it must be taken into consideration that, with increasing motor
speed, the absolute output volume, and consequently also the loop
gain of the volume flow control loop, rises, which may manifest
itself in an increase in the tendency to oscillate and, at worst,
result in an instability of the control. This risk must be taken
into consideration in particular whenever very small volume flows
have to be controlled, which is frequently the case where the power
lift and the trailer braking system are concerned.
Until now, allowance could only be made for these different
requirements if, for the setting of the control pressure difference
for providing the load-sensing control (volume flow control), a
compromise were made between the different operational requirements
of the units to be supplied, by the characteristic of the control
spring 26 being adapted to the predominantly occurring operational
requirements. However, it has been found that such a compromise
does not make allowance for the ever increasing requirements on the
dynamics of the control. For nowadays, even at low speed, such a
response capability of the controller is demanded that, at high
speeds, there is a tendency towards oscillations of the control
loop in the fine control range. Therefore, until now, there was no
alternative but to choose as great a control pressure difference as
possible, which however had the consequence that the design limit
for the lowest standby pressure level, i.e. that which it was
possible to achieve theoretically given the type of pump, could not
be achieved because, once fixed, the control pressure drop also
determined the standby pressure level. For this reason, until now
increased energy losses had to be accepted.
In addition, it has been found that it was, in principle, difficult
to provide adequately good control dynamics in the case of
consumers which operate in closed control loops with constant
reversal of the direction of action.
The invention is therefore based on the object of providing a
control for a load-dependently operating variable displacement pump
of the type described at the beginning which ensures an increased
stability at all operating points of the variable displacement
pump, even if consumers having extremely different requirements on
the control are supplied.
By the measures according to the invention, it is achieved that,
while operation is running, the control pressure drop can be
influenced selectively in such a way that the output capacity of
the control is adapted optimally to the respective operating point
of the consumer on the one hand and/or of the variable displacement
pump on the other hand. It is thus, for example, possible to fix
the control pressure difference for maximum speeds of the variable
displacement pump in such a way that no instability occurs. With
decreasing speed, the control pressure effective at the servo valve
is then influenced in such a way that the control pressure
difference is raised, in order to obtain a fast response behaviour,
i.e. good control dynamics as well as an adequate gain in the
load-sensing range, at slow speeds as well. The measures according
to the invention also create the prerequisite for it to be possible
to adapt the control pressure drop to the respectively most
important consumer of the mobile piece of machinery. At the same
time the additional advantage is obtained that the control spring
of the servo valve can be designed solely with a view to a standby
pressure which brings with it minimum possible pressure losses.
The arbitrary influencing of the control pressure effective at the
servo valve by means of a separately controllable actuating device,
together with the use of a servo valve, integrated into the
consumer supply line, with neutral centring position, results in
the additional advantage that the oil column in the load-indicating
line does not have to be relieved each time the servo valve passes
through the neutral position. As a result, a substantial
improvement in the control dynamics is obtained, which at the same
time drastically reduces the dynamic material stressing of the
components concerned, because a rapid loading and relieving of
these components no longer occurs. This is all the more significant
when the constant, even if brief relief of the load-indicating line
also brings with it a relief of the oil column between variable
displacement pump and servo valve. By the measures according to the
invention, the possibility is provided of leaving the oil column
between pump and servo valve uninfluenced upon passing through the
neutral position, thereby obtaining an improvement in the control
loop dynamics and at the same time a reduction in the dynamic pump
stressing.
It has been found that the separately controllable actuating device
can be readily designed in such a way that a setting of the control
pressure drop effective at the consumer between zero and the
maximum system pressure can be achieved. In this way, the entire
spectrum which is demanded by the widest variety of consumers of a
mobile piece of equipment, such as for example a farming tractor,
is successfully covered.
A particularly advantageous further development of the control is
obtained whenever the consumers are operated in closed control
loops which operate with constant reversal of the direction of
action. By the measures according to the invention, it is reliably
avoided that the oil column between variable displacement pump and
proportional directional valve is relieved upon each passage
through neutral. It has been found that consequently a considerable
improvement in the control loop dynamics can be achieved, there
being the additional advantage that the pump stressing is also
greatly reduced. The measures according to Patent Claim 2 create
the prerequisite for a stepless setting of the control pressure
drop effective at the consumer to be possible with a simple design
of the actuating device, by the orifice being preloaded with
pressures at different levels. In this way, it is possible to set
the control pressure drop selectively between zero and the maximum
system pressure. This means that it is possible, for example, in a
certain position of the actuating device, to operate the pump as a
constant pressure source, which is always advantageous whenever the
load-sensing control tends towards instability, which is the case
for example in the fine control range and at high drive speed.
In the simplest embodiment, the actuating device is designed as a
directional valve, allowing a step-by-step change in the control
pressure drop to be set. A simple stepless setting of the control
pressure drop is obtained if the directional valve is designed as a
proportional directional valve.
In this case, it is further of advantage to activate this
proportional directional valve electrically, because the complexity
of the circuitry of the hydraulic circuit can be further reduced as
a result. In addition, this type of activation has the additional
advantage, in particular in the case of electrically operated
consumers, that the setting of the effective control pressure drop
via the proportional directional valve is automatic, to be precise
can be performed according to the respective operating state of the
consumer on the one hand and/or of the variable displacement pump
on the other hand. However, it is to be emphasised that it is, of
course, also possible to reduce the tendency to oscillate of the
hydraulic load-sensing control and to raise the output capacity
even when the setting device is operated manually. In this way,
improving the fine control properties of the consumer and
selectively influencing and adapting the volume flow range in which
the consumer can be stably controlled to the other boundary
conditions can be accomplished.
A particular advantage of the subject of the invention is, in
addition, that the control spring of the differential pressure
controller no longer has to be matched to the maximum necessary
control pressure difference, because a variable support of this
control spring can be set during the course of operation by the
measures according to the invention. This has the consequence that
the standby pressure can be kept at a lower level with lower pump
stressing and with lower energy loss.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 diagrammatically shows a prior art load-sensing variable
displacement pump control loop;
FIG. 2 diagrammatically shows a control loop for a load-sensing
variable displacement pump according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
In FIG. 2, those components which are identical to the components
of the hydraulic circuit according to FIG. 1 are denoted by the
same reference symbols, so that a more detailed description of
these elements is not necessary. Differences from the hydraulic
circuit according to FIG. 1 exist in the area of the proportional
directional valve 60 and in the manner in which the control surface
20 of the differential pressure controller 30 is pressurized with
the control pressure x.
The servo valve 60, integrated into the consumer supply line 4, is
designed as a 4/3-way proportional directional valve, which is
electrically activated via the control signals i.sub.1. The three
switching positions are denoted by E, F and G. The proportional
directional valve 60 has an inlet connection 68, a tank connection
70 and two working connections 72 and 74, to which pump pressure is
alternately admitted. In this way, via the proportional directional
valve 60, a consumer can be operated in a closed control loop in
which a reversal in the direction of action is brought about by
constant reversing of the proportional directional valve 60. In the
neutral position F, shown in FIG. 2, which is maintained by the
centring springs 76 and 78 coming into effect, for example for the
case where the control signals i.sub.1 are not present or are of
the same size, all of the connections 68, 70, 72 and 74 are closed.
In the switching position E, a working line 80, leading to the
consumer, is connected to the inlet connection 68 of the consumer
supply line 4, while the further working line 82, coming from the
consumer, is relieved to the tank connection 70. In the switching
position G, a corresponding reversal of pressurization takes
place.
To ensure a predetermined pressure drop at the respectively
effective control edge of the proportional directional valve 60,
the respectively highest pressure in the working lines 80 and 82 is
picked off. For this purpose, two branch control lines 84 and 86
are led to a selector valve 88, from which a load-indicating line
90 leads to the control surface 20 of the differential pressure
control valve 30. Fitted into the load-indicating line 90 is a
restrictor 92, the function of which is explained in detail
later.
Downstream of the restrictor, there opens into the load indicating
line 90, carrying the control pressure x of the differential
pressure controller 30, a further control line 94, which leads to
an outlet connection 96 of a 3/3-way proportional directional valve
100. This servo valve is preferably activated electrically via the
control signal i.sub.2, so that it can be displaced steplessly
between the switching positions H, I and K, under the action of a
restoring spring 102. The proportional directional valve 100 has,
in addition, an inlet connection 98, which is pressurized with pump
pressure P via a control line branch 104. 106 denotes a tank
connection.
With such a design of the hydraulic circuit, the following mode of
operation is obtained:
As long as the servo valve 100 is kept in the switching position I
by the presence of a control signal i.sub.2, which keeps the force
of the spring 102 in balance, the circuit behaves like the
load-sensing control according to FIG. 1. This means that the
control spring 26 determines the respective control pressure drop
at the control edge of the servo valve 60, so that a volume flow
control can be carried out. Since, however, the load indicating
line 90 is not relieved to the tank each time the servo valve 60
passes through the neutral position F, the oil column between pump
2 and valve 60 is also not relieved in this case, as a result of
which an improvement in the control loop dynamics and a reduction
in the dynamic pump stressing occurs. On the other hand, without
additional precautions, the pump really should be operated in
standby operation at a higher pressure level, because the
respectively higher load pressure acts via the selector valve 88 on
the control surface 20. In order to eliminate the resulting
additional energy losses, for readying the variable displacement
pump 2 for standby operation, the servo valve 100 is displaced into
the switching position 100, in which a relief of the load
indicating line 90 to the tank T takes place. The swinging-back of
the pump, i.e. the reduction in output volume of the variable
displacement pump 2 already takes place at a pressure level which
is adequate to overcome the force of the control spring 26 by
acting of the differential control pressure valve 30 on the control
surface 22, so that the swinging-back process against the force of
a restoring spring 31 can be initiated by switching-through of the
control line 36 to the working chamber 32 of the variable
displacement pump 2.
In the third switching position H of the servo valve 100,
conversely an additional pressurization of the load indicating line
90 takes place, and consequently of the line carrying the control
pressure x effective at the differential pressure controller 30.
The control pressure x can in this way be raised to the extent that
the pump can be operated as a constant pressure source, which is
advantageous in particular whenever instabilities of the
load-sensing control are in evidence. This operating mode is of
advantage, for example, whenever the consumer is operated in the
fine control range and the variable displacement pump 2 is running
at high speed.
However, the switching positions H, I and K, described above, only
represent limit switching positions. The design as a servo valve
allows the spool valve to assume any intermediate positions, so
that, together with the restrictor or orifice 92, a stepless
setting of the control pressure x is possible within broad limits.
The control pressure drop effective at the consumer can in this way
be set between zero and the maximum system pressure. In order to
ensure that the consumer, for example a power lift, is capable of
holding the load in standby operation of the pump, it is of
advantage to integrate a hydraulically releasable non-return valve
in the working line leading to the consumer.
With the circuit according to the invention, which permits an
arbitrary influencing of the control pressure x effective at the
servo valve 30 during the operation of the consumers, the control
spring 26 can be designed in such a way that stable control
operation is obtained at full power. With falling speed, the
control pressure difference is then raised by increasing
pressurization of the control line 94 downstream of the orifice 92.
The orifice 92 is consequently increasingly preloaded with reducing
speed n of the variable displacement pump 2, so that, even at
extremely low speeds, an adequate gain of the load-sensing control
is obtained and a good speed of response, for example of the
steering system and of the suspension hydraulics, is ensured even
at low speeds. Dot-dashed lines indicate a signal flow, which is
passed on the one hand from the variable displacement pump 2 via
the speed signal n and on the other hand from the consumer to a
signal logic or a signal converter, via which the control signal
i.sub.2 for the servo valve 100 is adapted automatically to the
operating point of the pump 2 and/or of the consumer.
Of course it is possible, as a departure from the illustrative
embodiment represented, to perform the setting of the effective
control pressure drop manually, in order to reduce the tendency to
oscillate of the hydraulic load-sensing control and to increase the
output capacity. It is, for example, also possible to provide a
customary directional valve instead of a servo valve 100, in order
in this way to set an optimum control pressure difference only at
predetermined operating points.
The invention consequently provides a preferably electrohydraulic
control for a load-dependently operating variable displacement
pump, which is equipped with a differential pressure control device
for ensuring a predetermined pressure drop at the control edge of a
directional valve integrated into the consumer supply line. The
differential control pressure device has a servo valve, the valve
spool of which can be brought out of a first position, increasing
the output volume of the variable displacement pump, against the
compressive force of a spring and of a control pressure led off
from the supply pressure upstream of the directional valve, by the
pump pressure into a second position, reducing the output volume.
In order to be able to adapt the volume flow control to the
different requirements of various consumers, in particular of a
mobile piece of equipment, such as for example a farming tractor,
and thereby improve the quality of the control, the control
pressure effective at the servo valve is arbitrarily influenced by
means of a separately controllable actuating device, preferably
designed as a servo valve.
Various modifications in structure and/or function may be made by
one skilled in the art to the disclosed invention without departing
from the scope of the invention as defined by the claims.
* * * * *